1. Field of the Invention
The present invention relates to a contact construction for DC loads and a switching device having the contact construction for DC loads.
2. Description of the Related Art
In existing switching devices such as relays having a stationary contact and a movable contact which are opposite to each other, silver-tin oxide-indium oxide-based contacts (hereinafter referred to as the AgSnO2In2O3-based contacts), silver-tin oxide-based contacts (hereinafter referred to as the AgSnO2-based contacts), silver-nickel-based contacts (hereinafter referred to as the AgNi-based contacts), silver-zinc oxide-based contacts (hereinafter referred to as AgZnO-based contacts) have been used as contact materials. In general, each of the contact materials is individually used as a contact material common to a movable contact and a stationary contact. In such switching devices, attempts to cope with higher voltages have recently been made. In general, means such as enlarged contact-to-contact gaps are needed to realize switching devices capable of coping with higher voltages, but if switching devices having reduced sizes are to be realized, the contact-to-contact gaps are impossible to enlarge beyond approximately 1 mm. However, if the contact-to-contact gap of a switching device is simply set to approximately 1 mm, an arc remains for a comparatively long time, and if the arc continues to remain for a period of, for example, 100 ms or longer, the problem that cut-off failure occurs in the switching device arises.
Another problem occurring when an arc remains for a long period is that the surfaces of contacts of a switching device are heated to high temperatures so that locking or deposition occurs between the contacts or burning and destruction of the contacts occur to impair the life of the switching device. This problem is particularly remarkable in switching devices of the type which cut off high-capacitance loads. Locking is the phenomenon that a depression and a projection which are formed by the transfer of a contact material from one of the contacts to the other are caught to disable or delay the release of the movable contact from the stationary contact. Deposition is the phenomenon that owing to the melting of the contact surfaces, the movable contact and the stationary contact stick to each other, so that their release is disabled or delayed.
To achieve a long life of the switching device, there are various available methods such as a method of improving the heat resistance of the contacts as by enlarging the contacts and contact parts to which the contacts are secured and increasing the heat capacity of the switching device, a method of enlarging the contact-to-contact gaps to prevent abnormal continuation of an arc, and a method in which release force acting between the contacts is set to a large force so that even if the contacts adhere to each other by deposition, they can be peeled off each other. However, these methods cannot satisfactorily achieve a long life of the switching device, and incur an increase in the size and/or cost of the switching device.
To cope with this problem, it is known to use a method of producing a magnetic field between the contacts by means of a magnetic unit such as a permanent magnet. If a magnetic field is produced between the contacts, a Lorentz force acts on an arc, and the arc is significantly driven (travels) between the contacts in accordance with the Fleming's left hand rule. Accordingly, the concentration of the arc on the surfaces of the contacts is avoided and the arc is easily cut, so that a long life can be achieved.